The present invention relates to a line pressure control for an automatic transmission, and more particularly to a line pressure control when the automatic transmission undergoes a downshift operation.
An automatic transmission is shiftable between two gear positions by switching friction element or elements which are hydraulically activated by a hydraulic fluid pressure often called a line pressure.
During a downshift operation, a friction element on the high speed side is released to render the transmission operable in neutral state, allowing the engine to increase its revolution speed, and then another friction element on the low speed side is engaged. In order to shorten a period of time required for such a downshift operation, it is effective to decrease the magnitude of line pressure at an initial stage of the downshift operation so as to quickly release the friction element on the high speed side. However, if the timing when the line pressure is increased to the original level is retarded, the another friction element on the low speed side slips for a prolonged time. Thus, this measure of decreasing the line pressure at the initial stage of such downshift operation is difficult to be put into practice owing to the difficulty of managing the timing when the line pressure is increased to the original level.
An object of the present invention is to provide a line pressure control wherein the timing when the line pressure is increased to restore its original level is appropriately managed, so that the line pressure is allowed to be decreased sufficiently.